Optical communications link

ABSTRACT

The present invention provides an optical communications link having at least one optical fiber, in particular for communications transmission, where the optical fiber is repeatedly bent. The fiber sections having a right-hand and left-hand curvature are distributed in such a way over the communications link that the average torsion of the fiber is approximately zero. The communications link in accordance with the present invention is compact, flexible, and variable in length. In addition, it reduces the sensitivity of the polarization state of the optical signal to changes in the form of the communications link.

FIELD OF THE INVENTION

The present invention relates to a movable optical communications linkhaving at least one optical fiber; in particular, for use intransmitting information or performing interferometric measurements.

BACKGROUND OF THE INVENTION

Optical fiber links used to transmit information via light havesignificant advantages, both for long transmission links intelecommunications, as well as for short transmission links insidebuildings, vehicles, and machines, not to mention in electroniccalculating machines, since they ensure high data transmission densityaccompanied by low power losses. Due to their thin, flexible, butmechanically very durable construction, incoming optical fiber lines andoutgoing optical fiber lines are beneficial, particularly for connectingoptical sensors for measuring physical parameters, such as pressure andtemperature, etc. In addition, unlike electrical connections, suchoptical fiber links cannot cause any electrical sparkovers or shortcircuits. The high transmission capacity of the optical fibers makes itpossible to modify or replace the sensors and measuring devices withouthaving to replace the communication links. This can result inconsiderable cost savings in vehicles, buildings, machines, orproduction facilities. There is often the need for optical fiber linksto be mechanically movable, such as when installed in robots. Further,in buildings and vehicles, one frequently encounters motion amongcomponents due to strain or expansion.

Therefore, optical fiber links for transmitting information are alwaysof great benefit when there is a need to transmit high informationdensities and a mechanically flexible connection is required, since thedistance between the sender and receiver of the information varies as afunction of time.

A difficulty that arises that significant changes in the position of thetransmitter and/or of the receiver, and, in particular, in theirrelative distance spanned by optical communication links constituted assimple cable, can cause the entire system, such as a remote-controlledrobot, to be obstructed by the requisite reserved length of cable.Individual components, which communicate with one another via an opticalcommunications link, can become mechanically blocked by loops of cable.Another difficulty is that one can end up with a “cable salad”.

Another difficulty encountered in response to variations in the positionand distance of transmitters and/or receivers involves the nature of theoptical transmission signal.

In communications transmissions of high quality and transmissionfrequency, it is necessary to control the polarization state of theoptical information flow in the optical fiber, as well as in the otheroptical components. In the case of coherent transmissions, for example,phase-coherent mixing of the optical information flow with other lightsources must be carried out. Such phase-coherent mixing is only optimalwhen the polarization states are substantially identical. When workingwith high bit-rate transmissions, the polarization mode dispersion ofthe fibers limits the reception quality, and transmission frequency canonly be increased by carefully controlling the polarization. In manyother optical components as well, the performance is a function of thepolarization of the light.

Generally, the polarization state of the light in an optical fiber isnot constant. Each glass fiber has a certain elliptical birefringence,so that the polarization of the light continually changes in the fiber.This variation propagates through to the end of the fiber, and, since itis dependent upon the spatial geometry of the fiber curve, thepolarization state at the output end of a moving fiber varies with themotion.

In known methods this polarization effect can be avoided in that theoptical communications transmission takes place in one of the intrinsicmodes of a polarization-maintaining fiber. Thesepolarization-maintaining fibers are characterized by pronouncedbirefringence, so that there is virtually no coupling over between thetwo polarization modes in the fiber. Since a change in the polarizationof the light in an optical fiber is a phase shift effect between theintrinsic modes of the light, the polarization mode dispersion does notoccur when the light in the fiber propagates through permanently in oneintrinsic mode only.

The drawback of this method is that the polarization-maintaining fibersare expensive. Moreover, the light must be launched at the input ends ofthe polarization-maintaining fiber in a defined polarization state.

SUMMARY OF THE INVENTION

The present invention provides an optical communications link which canovercome the above-described difficulties and problems. To ensure a hightransmission quality, the polarization state of the light should notdepend substantially on changes in the form of the communications linkand, therefore, on changes in the position of the transmitters andreceivers. In addition, the communications link should be easilyadaptable to changes in form, in particular to variations in length,but, it in this context, always be characterized by a straightforwardarrangement.

In the present invention, the optical communications link having atleast one optical fiber, in particular for communications transmission,where the optical fiber is repeatedly bent or curved and, in theprocess, can be wound in a helical shape, alternating as a right-handand left-hand helix, fiber sections having a right and left curvaturebeing distributed in such a way over the communications link that theaverage torsion of the fiber over the communications link isapproximately zero.

The optical communications link of the present invention can be designedso that the sensitivity of the polarization state of the opticaltransmission signal to changes in the form of the communications linkand, i.e., of the optical fibers, is substantially compensated. This isassured by the present invention in that the optical fiber is repeatedlybent, fiber sections having left-hand and right-hand curvature beingdistributed in such a way over the communications link that the averagetorsion of the fiber over the communications link is more or less zero.Preferably, this also holds for individual subsections of the fiber, sothat left and right curvatures are uniformly distributed over the fiber.By preference, the fiber is wound in a helical shape, alternating with aright-hand and left-hand helix. Mixed forms having an even meander shapeare also possible.

The present invention concerns the motion- and form-dependentbirefringence of an optical fiber: the linear birefringence is heavilydependent upon the ellipticity are the fiber core, less heavilydependent upon the bend of the fiber, and hardly dependent upon thehelical winding, given a large radius of the fiber. In contrast, thecircular birefringence is hardly dependent upon the ellipticity of thefiber core and on the curve of the fiber, on the other hand, veryheavily dependent upon the helical winding of the fiber. The main reasonfor the form dependency of the polarization state at the output end ofan optical fiber is the considerable dependency of the fiber's opticalactivity upon the exact form of its helical windings. In the firstapproximation, this effect is achromatic and does not result in anypolarization mode dispersion. It is caused by one of the so-calledoptical Berry phases, the “spin redirection phase” (R. Y. Chiao, Y. S.Wu, Phys. Rev. Lett. 57, 933 (1986)). This Berry phase (or geometricphase) is a phase effect produced by the structure of the fiber's spacecurve and not by an optical path, as is the case with the normal dynamicphase of the light. Nevertheless, with respect to interference of thelight, geometric phases have the same properties as the normal dynamicphase.

The size of the spin redirection phase in a helically wound fiber isequivalent to the solid angle Ω that the k vector (k corresponds to thepropagation constant β in the technical literature) wraps around on thesphere of the light-propagation orientations in the counterclockwise-direction when the light in the fiber is directed through ahelical winding. The spin redirection phase is additive and changes itsoperational sign when the helical direction of the fiber changes, e.g.,from the left-hand to the right-hand helix.

To minimize this form-dependent polarization effect, the fiber must bemade up of wound fiber sections having alternating winding directions.As an example, the fiber sections are alternately wound to the right andto the left, the space angle, which wraps around the k vector in theleft-hand wound sections, being equivalent to the space angle that the kvector wraps around in the right-hand wound sections. In the simplestcase, the fiber alternately follows a right-hand and then a left-handhelix, each time with an equivalent length and winding; or right-handand left-hand wound fiber sections of a fixed length alternate with eachother.

In an embodiment of the present invention, to reduce the polarizationdependency of changes in the form of the fiber link, the sections havingright-hand and left-hand helical winding of the fiber should bedistributed over the fiber in such a way that, in response to an alteredfiber form, the changes dΩ_(i) in the solid angles Ω_(i) of the kvectors in the i-th fiber section add up to zero, thus to ΣdΩ_(i)=0.

The variation in the polarization of an optical signal at the output endof a moving optical communications link having one optical fiber isadvantageously reduced in that the optical fiber is repeatedly bent,fiber sections having a right and left curvature being distributed insuch a way over the communications link that the average torsion of thefiber over the communications link is approximately zero.

In order to minimize the variation in polarization in the case ofchanges in the form of only one fiber section, the optical fiber ispreferably bent in such a way that the torsion of the subsectionaveraged over subsections of the communications link is approximatelyzero. In this context, a subsection is a fiber section which is at leastsufficiently long to contain right-hand and left-hand fiber segments,e.g., two successive, individual right-hand and left-hand windings, thetorsion of the two sections canceling each other.

In an embodiment of the present invention, the optical fiber can becoiled with alternating winding direction around an even number of,preferably two, side-by-side carrier elements. In this context, one or aplurality of left-hand windings around one of the carrier elements canfollow the corresponding number of right-hand windings around anothercarrier element.

Another embodiment of the communications link provides for two helicallywound optical fibers (1, 3, 6) having different winding directions inorder to direct the light in the forward and return directions.

In this further embodiment, the communications link has at least twohelically wound optical fibers having different winding directions todirect the light in the forward and return directions. In this context,both optical fibers can be advantageously wound around the same carrierelement, the outer winding of the two windings having a somewhat largercoil pitch, so that, in terms of absolute value, the torsion of theforward and return line is more or less equivalent, but with differentoperational signs.

In a further embodiment of the present invention, the communicationslink permits the transmission of information in moving fibers, with asubstantially reduced polarization variation at the output end.

To minimize the effects of the bending- and stress-induced birefringenceof the fiber material on the polarization state of the transmissionsignal, too small of a winding radius for the optical fibers should notbe selected. Preferably, the winding radius for the optical fibersshould be to at least about 2 cm, or to at least about 3 cm.

In a further embodiment of the present invention, the optical fiber canbe joined to an elastic carrier material, which, in response tomechanical loading, can permit a change in the form of the transmissionline and, in response to the lack of a mechanical load, can retain theoptical fiber in its initial curved form.

This communications link of an embodiment of the present invention canmake it possible to establish a connection that is compact, yet movableand variable in length, for transferring optical data between atransmitter and a receiver. In this manner, any mechanical hindrance tothe overall device, including the transmitter, receiver andcommunications link, can be minimized. And, the output signal can besubstantially insensitive to any changes in the form of thecommunications link.

In a further embodiment of the present invention, the optical fiber canbe wound in a helical shape, e.g., in the manner of a telephone cable.In response to stress in the longitudinal direction of the helix, i.e.,of the meander shape, the communications link can be pulled apart in anaccordion-like fashion, and, in response to cancellation of the stress,again assumes its compact, initial form.

In another embodiment of the present invention, the optical fiber can bewound around at least one elongated carrier element, such as a cylinder.The carrier element is preferably flexible. As an example, the carrierelement is a flexible bar.

To realize and stabilize its curved form, the fiber is preferablysecured to the carrier element in such a way that it is movable in itswound form, but remains stabilized on the carrier element, e.g., in thatit is flush mounted on the carrier element or embedded between thecarrier element and a cladding material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of the transmission lines according to anembodiment of the present invention for reducing the influence of formon the polarization of the output signal;

FIG. 2A shows an example of the transmission lines according to anembodiment of the present invention for reducing the influence of formon the polarization of the output signal;

FIG. 2B shows an example of the transmission lines according to anembodiment of the present invention for reducing the influence of formon the polarization of the output signal; and

FIG. 3 shows an example of the transmission lines according to anembodiment of the present invention for reducing the influence of formon the polarization of the output signal.

DETAILED DESCRIPTION

FIGS. 1 through 3 illustrate examples of transmission lines according toembodiments of the present invention which are compact, movable, andflexible. Furthermore, the transmission lines are designed to minimizethe influence of the transmission lines' form on the polarization of theoutput signal. Thus, they are especially suited for linking opticaltransmitters and receivers, which are movable with respect to oneanother, for purposes of data communications.

The top part of FIG. 1 shows a detail of such a communications link,which is made up of a cylinder 2, as a carrier material or carrierelement, and of an optical fiber 1. Optical fiber 1 is helically woundaround cylinder 2, the direction of the helical winding changing, forinstance, in the middle of the cylinder at point B. Thus, in the leftpart of the communications link, the torsion of the optical fiber isnegative, in the right part, positive, so that the average torsion ismore or less zero.

To change the direction of the helical winding on a cylinder, an arc Bshould be wound. This arc can be secured, together with the remainingright- and left-hand winding, for example, by adhesive or by tying it tothe cylinder, since otherwise it would become detached.

To manufacture a long communications link, a plurality of line segmentscan be joined to one another, as shown in FIG. 1. The depicted fibersegment is then a subsection, in which the average torsion isapproximately zero.

In the lower part of FIG. 1, the k vector of the light launched into thefiber and the corresponding solid angle Ω are shown. If r(s) denotes thespace curve described by the fiber as a function of the arc length s,then solid angle Ω can be derived as a measure for the Berry phase fromthe torsion τ of the space curve, as follows (s₁, s₂ denote thebeginning and end, respectively, of the fiber):∫_(s1)^(s2)τ(s)  s = Ω ∝ Φ_(Berry),

where k(s1)=k(s2)

Two further embodiments of communications links or of sections thereof,in accordance with the present invention, are shown in FIG. 2. In FIG.2A, optical fiber 3 is doubly wound over two cylinders 4, 5. Aroundcylinder 4, fiber 3 describes a left-hand winding (L), around cylinder5, a right-hand winding (R). By alternating the two cylinders, aright-hand helical winding and a left-hand helical winding can alwaysalternate with one another.

In this context, glass fiber 3 is embedded, similarly to a telephoneline, in a material which has dimensional stability, but is highlyelastic, so that the incoming line can be pulled apart in accordion-likefashion, but contracts again when the tensional force subsides. Inaddition, cylinders 4, 5 can themselves be resilient to facilitate alateral motion of the communications link.

The optical signal can be conducted in the reverse direction through thesame glass fiber, however, over a different spectral channel, forexample. Since the geometric phase is achromatic, and a right-hand helix(or left-hand helix) remains a right-hand helix (or left-hand helix)when it is propagated through in the opposite direction, the samecompensation effect occurs for the optical forward and reverse line asdoes for the form-dependent polarization fluctuations in the presentinvention.

In place of two cylinder windings of fiber 3 as shown in FIG. 2A, thefiber 6 can also be routed over more cylinders, i.e., four cylinders 7,8, 9, 10. This is shown in FIG. 2B. In the case of FIG. 2B, right-handand left-hand loops alternate, each characterized by R or L.

It is also possible for a plurality of left-hand loops to follow aplurality of right-hand loops in that the fiber is repeatedly woundaround a cylinder before it is routed to the next cylinder with anopposite winding direction. It is crucial here that the formulaΣdΩ_(i)=0 remain satisfied, and that the torsion of the entire opticalfiber be compensated.

The achromaticity of the geometric phase makes it possible to use bothwhite light sources, as well as more or less monochromatic lightsources.

In the case that the light is directed in the forward and reversedirection through the same communications link, one can configure twocylinder windings side-by-side, one of these, a right-hand helix,functioning as an incoming line, and the other, a left-hand helix, as areturn line. The flexible claddings, which determine the form elasticityof the line, can be configured separately from one another. However, theflexible claddings of an embodiment of the present invention, aredesigned as contiguous claddings. This can prevent them from separatingfrom another, thereby permitting them to jointly participate in themotion of the line, substantially identically.

In such a case of a single right-hand helix as a forward (reverse) lineand of a single left-hand helix as a reverse (forward) line, the twoelastic helical windings 11, 12 can also be wound, one over another, ona single cylinder 13, as shown in FIG. 3. Since the outer winding has asomewhat larger diameter, its pitch should be somewhat greater than thatof the inner winding, in order to satisfy the condition ΣdΩ_(i)=0.

The incoming and outgoing lines described here can be used, for example,to freely span the distance between a stationary base station, e.g., themeasuring or control unit, and the movable sensor, e.g. a telephonereceiver or another sensor; or they are supported by tubes or wiresusing pull or tension rollers. In this manner, the dependency of thepolarization of the transmitted light on the motion of the line isreduced. It is also beneficial to use lines of the described type havingalternating helical winding to provide movable connections of variouscable links in the telecommunication nodal points with the aid of shortglass fiber lines equipped with plug connectors. These freely movablelines then introduce a substantially smaller time-related polarizationchange into the information flow of the transmission link than do thecustomary loop-type lines. Moreover, they reduce the “cable salad”.

The present invention has industrial applicability in all fields inwhich optical signals are transmitted via optical fiber links.Embodiments of the present invention can be used for systems havingtransmitters and receivers of optical signals which experience relativepositional changes, and where the quality of the transmission signal isoften degraded by changes in the form of the transmission link.

What is claimed is:
 1. An optical communications link comprising: anoptical fiber for transmitting information, the optical fiber having aplurality of fiber sections, each fiber section of the plurality offiber sections being configured to have at least one of a right-handcurvature and a left-hand curvature, the optical fiber being bentrepeatedly so that the plurality of fiber sections having a right-handcurvature and a left-hand curvature are distributed over the opticalcommunications link so that an average torsion of the optical fiber overthe optical communications link is about zero, wherein the optical fiberis bent so that a torsion of the fiber section of the plurality of fibersections averaged over a total subsections of the communications link isabout zero.
 2. An optical communications link comprising: an opticalfiber for transmitting information, the optical fiber having a pluralityof fiber sections, each fiber section of the plurality of fiber sectionsbeing configured to have at least one of a right-hand curvature and aleft-hand curvature, the optical fiber being bent repeatedly so that theplurality of fiber sections having a right-hand curvature and aleft-hand curvature are distributed over the optical communications linkso that an average torsion of the optical fiber over the opticalcommunications link is about zero, wherein the optical fiber is wound ina helical shape, alternating with a right-hand and left-hand windinghelix, wherein the right-hand and left-hand winding helix includes aright-hand helical winding and a left-hand helical winding so that theright-hand helical winding follows and alternates with the left-handhelical winding, a right length of the right-hand helical windingcorresponding to a left length of the left-hand helical winding.
 3. Anoptical communications link comprising: an optical fiber fortransmitting information, the optical fiber having a plurality of fibersections, each fiber section of the plurality of fiber sections beingconfigured to have at least one of a right-hand curvature and aleft-hand curvature, the optical fiber being bent repeatedly so that theplurality of fiber sections having a right-hand curvature and aleft-hand curvature are distributed over the optical communications linkso that an average torsion of the optical fiber over the opticalcommunications link is about zero, and an elastic carrier material, theelastic carrier material being joined to the optical fiber so that aform change of a transmission line is permitted and so that in responseto no mechanical load the transmission line retains the optical fiber inits initial curved form, the transmission line configured as a pluralityof the optical fibers.
 4. An optical communications link comprising: anoptical fiber for transmitting information, the optical fiber having aplurality of fiber sections, each fiber section of the plurality offiber sections being configured to have at least one of a right-handcurvature and a left-hand curvature, the optical fiber being bentrepeatedly so that the plurality of fiber sections having a right-handcurvature and a left-hand curvature are distributed over the opticalcommunications link so that an average torsion of the optical fiber overthe optical communications link is about zero, a carrier element, thecarrier element being an at least one of an elongated carrier elementand a cylinder, the optical fiber being wound around the carrierelement, wherein the at least one of the elongated carrier element andthe cylinder is flexible.
 5. An optical communications link comprising:an optical fiber for transmitting information, the optical fiber havinga plurality of fiber sections, each fiber section of the plurality offiber sections being configured to have at least one of a right-handcurvature and a left-hand curvature, the optical fiber being bentrepeatedly so that the plurality of fiber sections having a right-handcurvature and a left-hand curvature distributed over the opticalcommunications link is about zero; and a carrier element, the carrierelement being an at least one of an elongated carrier element and acylinder, the optical fiber being wound around the carrier element,wherein the optical fiber is secured to the carrier element so that theoptical fiber is movable and still stabilized on the carrier element. 6.An optical communications link comprising: an optical fiber fortransmitting information, the optical fiber having a plurality of fibersections, each fiber section of the plurality of fiber sections beingconfigured to have at least one of a right-hand curvature and aleft-hand curvature, the optical fiber being bent repeatedly so that theplurality of fiber sections having a right-hand curvature and aleft-hand curvature are distributed over the optical communications linkso that an average torsion of the optical fiber over the opticalcommunications link is about zero, a carrier element, the carrierelement being an at least one of an elongated carrier element and acylinder, the optical fiber being wound around the carrier element, anda cladding material, the optical fiber being at least one of flushmounted on the carrier element and embedded between the carrier elementand the cladding material, wherein the optical fiber is secured to thecarrier element so that the optical fiber is movable and stillstabilized on the carrier element.
 7. An optical communications linkcomprising: an optical fiber for transmitting information, the opticalfiber having a plurality of fiber sections, each fiber section of theplurality of fiber sections being configured to have at least one of aright-hand curvature and a left-hand curvature, the optical fiber beingbent repeatedly so that the plurality of fiber sections having aright-hand curvature and a left-hand curvature are distributed over theoptical communications link so that an average torsion of the opticalfiber over the optical communications link is about zero, and a carrierelement, the carrier element being an at least one of an elongatedcarrier element and a cylinder, the optical fiber being wound around thecarrier element, wherein the optical fiber is coiled with an alternatingwinding direction around one of two carrier elements disposedside-by-side and an even number of the carrier elements disposedside-by-side.
 8. An optical communications link comprising: an opticalfiber for transmitting information, the optical fiber having a pluralityof fiber sections, each fiber section of the plurality of fiber sectionsbeing configured to have at least one of a right-hand curvature and aleft-hand curvature, the optical fiber being bent repeatedly so that theplurality of fiber sections having a right-hand curvature and aleft-hand curvature are distributed over the optical communication linkso that an average torsion of the optical fiber over the opticalcommunications link is about zero, and a carrier element, the carrierelement being an at least one of an elongated carrier element and acylinder, the optical fiber being wound around the carrier element,wherein a left-number of the left-hand windings around a first of thecarrier elements is equivalent to a right-number of the right-handwindings around a second of the carrier elements.
 9. An opticalcommunications link comprising: a first optical fiber for transmittinginformation, the first optical fiber having a first plurality of fibersections, each fiber section of the plurality of fiber sections beingconfigured to have at least one of a first right-hand curvature and asecond left-hand curvature, the first optical fiber being bentrepeatedly so that the first plurality of fiber sections having a firstright-hand curvature and a first left-hand curvature is distributed overthe optical communications link so that a first average torsion of thefirst optical fiber over the optical communications link is about zero;a second optical fiber for transmitting information, the second opticalfiber having a second plurality of fiber sections, each fiber section ofthe second plurality of fiber sections being configured to have at leastone of a second right-hand curvature and a second left-hand curvature,the second optical fiber being bent repeatedly so that the secondplurality of fiber sections having a second right-hand curvature and asecond left-hand curvature are distributed over the opticalcommunications link so that a second average torsion of the secondoptical fiber over the optical communications link is about zero; thefirst and second optical fibers being helically wound and havingdifferent winding directions so that the first optical fiber directslight in a forward direction and the second optical fiber directs lightin a return direction.
 10. The optical communications link as recited inclaim 9, wherein the first optical fiber and the second optical fiberare wound around the same carrier element producing an outer winding ofa larger coil pitch than an inner winding so that a first torsion of aforward line of the first optical fiber is similar in magnitude to asecond torsion of a return line of the second optical fiber, the firsttorsion and the second torsion having different operational signs. 11.The optical communications link as recited in claim 9, wherein theoptical fiber has a winding radius of one of greater than 2 cm andgreater than 3 cm.